1. Field of the Invention
The present invention relates to an image compression method, an image restoration method, a program and an apparatus which compress a bit-mapped image so as to be transferred to a printer and the like, and after that, to be restored, and in particular, it relates to an image compression method, an image restoration method, a program and an apparatus which are capable of super-compressing an image change such as a gradation.
2. Description of the Related Art
In recent years, in general, an information technology device such as a personal computer and the like has become widespread, and has come to be used for various purposes. Particularly, there has been often the case where an image processing is performed by the PC, and a variety of processing has been performed where a printer is connected by the PC to print an image or an image data is transferred through a network such as an Internet and the like. However, the more complicated the image becomes, the more it takes time to transfer the image data from the PC to the printer or to transfer the image data through the network. Thus, it is desired that an image data be compressed and transferred. Hence, in case a bit-mapped image is directly transferred to a printer having no font and a printing is performed, the image data is compressed, so that an amount of the data to be transmitted is reduced, thereby saving a transferring time. In this case, though there are colored and gray scale data in the image data, a compressing method of a binarized bit-mapped image which binarizes these image data will be described below.
As for the conventional compressing method of the binarized bit-mapped image, there are MH (Modified Huffman), MR (Modified Read), MMR (Modified Modified READ), JBIG (Joint Bi-Level Image Experts Group) and the like, which are widely used for a FAX communication and an image filing.
Since these compressing methods are in heavy use of the bit processing and are not suitable for high speed encoding processing, the [compressing method of bit mapped-data and the compressing apparatus thereof] aiming at high speed processing has been proposed (by Japanese Patent Application Publication No. 3278298). According to this image compressing method, there is a problem that, while a bit-map mainly composed of characters can be effectively compressed, on the other hand, the compression efficiency of the dithered image is low. Hence, the present inventor has proposed a compressing method (Japanese Patent Application Laid-Open Publication No. 2001-388931), which enhances the compression efficiency of a uniform dithered image in which the same patterns continuously appear repeatedly.
However, the compression method proposed by the present inventor has caused a problem that, while the uniform dithered image in which the form of the pattern does not change but repeats itself can be effectively compressed, on the other hand, the dithered image in which the form of the pattern continuously changes similarly to the gradation cannot be compressed at all.